Conductive pillars may be formed on a semiconductor substrate in order to provide a physical and electrical connection point for external connectors. Generally, these conductive pillars are formed through a top passivation layer of the semiconductor substrate, thereby providing an external connection to the active devices formed on the semiconductor substrate. The conductive pillars are formed in a cylindrical shape in order to accommodate later formed connections, such as a spherical conductive bump.
The conductive bump may be formed on the conductive pillars from a connecting material such as solder. Typically, the conductive bump is placed onto the conductive pillars and then heated such that the conductive bump is partially liquefied and reflows into a bump shape. Once formed, the conductive bump may then be placed into contact with a separate substrate such as, for example, a printed circuit board or another semiconductor substrate. After the conductive bump has been placed in contact, the conductive bump may again be reflowed in order to bond the conductive bump to the separate substrate, thereby not only providing an electrical connection between the semiconductor substrate and the separate substrate, but also providing a bonding mechanism between the semiconductor substrate and the separate substrate.
However, for such a process to be reliable, the amount of conductive material must be precisely controlled when it is placed onto the circular conductive pillars. If there is an excessive amount of conductive material, there is an increased risk that conductive bumps that are adjacent to each other could unintentionally make contact and bridge during the reflow process, providing an undesired short-circuit. Conversely, if there is an insufficient amount of conductive material, there is an increased risk that there is not enough conductive material to provide a sufficient connection between substrates, thereby leading to an increased risk of a cold joint.
Additionally, the interface between the conductive bump and the circular conductive pillar is a vulnerable spot for cracks that may be initiated by the bonding process. This vulnerability could be further aggravated if the sidewalls of the conductive pillar is fully exposed to an ambient atmosphere and allowed to excessively oxidize, thereby increasing the risk of delamination between the conductive pillar and the underfill.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.